1. Field of the Invention
The present invention relates generally to cargo containers, and more particularly concerns a blast-resistant cargo container that is capable of substantially confining an explosive blast within the cargo container for protecting a carrier such as an airplane.
2. Description of Related Art
Conventional cargo containers are typically designed to have a frame, and panels attached to the frame so as to define a chamber interior for receiving goods. There are many kinds of available cargo containers having different sizes and configurations in order to meet practical needs, wherein an air cargo container is a kind to be used for transporting goods via an airplane.
Recently, airplanes have become a primary target for terrorist attacks, and many people have lost their lives in plane crashes due to terrorist bombing. Therefore, the Federal Aviation Administration (FAA) and major airline companies all over the world are forced to enhance security checks at the Custom of an airport in order to prevent explosives being smuggled on board. However, small plastic explosives are difficult to be detected despite current technology and are very likely to pass through the security checks. In the tragedies of Pan Am103 1998 and UTA Flight 772 1989, the explosives were smuggled on board the jets and caused plane crashes that resulted in loss of hundreds of lives and properties. Therefore, to prevent these kinds of tragedies from happening, a lot of efforts have been made in the field of blast-resistant containers.
According to analysis and experiment research, an explosive blast destroys the cargo container in two stages. In the first stage, shock waves are generated and impact the cargo container in a short duration. In the second stage, the succeeding much longer, more uniform and much lower magnitude explosive pressure exerts on the cargo container. The failures of both these two stages must be countered so as to make a container blast-resistant. During the moment of the explosion, the pressure at the center of the blast can be hundreds of thousands times of atmospheric pressure. Fortunately, the very great shock pressure is not definitely to cause structural failure in general due to it being very short duration and being a very local loading to a container. With the fast propagation and rapid decay of the shock waves, the ensuing pressure exerted on the panels is still no less than several ten times of the atmospheric pressure. With reflection and diffraction of the waves, the pressure becomes steady and its magnitude is much less than the shock waves. However, the pressure is still tens times greater than the payload of the conventional cargo container. Therefore, the conventional cargo container as shown in FIG. 14 is vulnerable to the explosive blast.
In order to overcome the mentioned problems, several blast resistant techniques are applied to the air cargo container. The first category utilizes the venting method, such as found in U.S. Pat. No. 5,195,701, wherein an explosive propels the venting device to pierce the wall of the fuselage of an airplane and this allows venting of the shock waves and high pressure to the exterior of airplane. For that it supposed to resist so high pressure (the pressure of shock wave may exceed one million pounds per square inches) is impractical. So that the high blast pressure is thus vented in a controlled manner outside the air cargo container to prevent a total destruction. Nevertheless, the high blast pressure and its carrying broken materials with high kinetic energy are still possible to puncture the fuselage wall of the airplane ultimately and this results in a crash. Although puncturing a wall may not usually be serious enough to cause a crash, damage to the airplane is still very costly to repair, and the time that the airplane is grounded is very expensive in lost income. Therefore, it is considered to be impractical to use the venting method to deal with the explosive blasts.
The second category utilizes the rigid confining method. With reference to FIG. 14, an air cargo container (40) designed according to this method is shown and has a rigid frame (41). Panels (42) made of energy-absorbing material are mounted onto the frame (41). The panels (42) must be inordinately thick enough so as that the wall can absorb destroying energy and to withstand the pressure of the explosive blast, and to further confine the pressure inside the air cargo container (40). However, in practice, what really destroys the structure of the air cargo container (40) is high stress, and high energy is not definitely the important factor for that it doesn't always induce high stress in a structure system. Increasing the thickness of the panels (42) not only increases the cost, but also increases bending stress when encountering a blast inside the container. As a result, the increasing in the weight of the container is not acceptable by the aviation industry because the weight increase of the container means a lot more expenses will thus be incurred. Therefore, in order to overcome the high stress, the air cargo container (40) has to be constructed so heavily that it is not feasible to be carried by the airplane.
Based on more detail studies of a container structure we also found: Even though the container panels are made of high strength materials, if the structure is not adapted, the panels and their edges nearby can be damaged by the explosive blast. I.e., even there are high strength materials for panels, if there is no appropriate structure layout to make the structure stress redistribution in the edges nearby during blast loading, the pressure from explosion inside the cargo container will generate tremendous bending stress on the container edges and causes serious destruction.
U.S. Pat. No. 6,237,793 basing on the similar recognition: “the seams along the frame where the panels are connected are typically the weakest point of the container in an explosion”, it used flexible (cloth) panels to wrap the rigid frames to form a light weight blast-resistant air cargo container. In this patent, a rather rigid frame system offers the stiffness of the container for operation of loading/unloading goods as usual and the very flexible panels, which are cloth made of high strength and light weight composites and never induce great bending stress, can deform to a spherical-like shape and take the great pressure under a blast. But it is not an idea choice for a cargo container with too flexible side panels in general.
The explosive containment device of U.S. Pat. No. 6,196,107 is originated from the conventional bomb containment vessel, which is spherical or cylindrical shell made of steel, and is a box-like shell having flat side panels and their transition portion (edges). So that it is a frameless design. In this patent, the entire continuous shell made of ductile material (steel) can be plastically deformed greatly and can take the great pressure under a blast. But it is too heavy (several times greater than light weight blast-resistant air cargo container, mainly due to its high density metal material) and absent the space to be put many packages for real use. The bending stiffness of this structure in the edge nearby is too great to deform fully to a spherical-like shape in general. There are no mechanisms to vary the bending stiffness in the edge nearby and replace light weight side panels.